Generally, the combustion of fuels containing sulfur, such as sulfur-containing coals, produces sulfur dioxide which had previously been discharged into the atmosphere. Several systems have now evolved for removing sulfur dioxide from stack gases, one of which is to pass the gases through a wet scrubbing system. Such wet scrubbing systems may use slurries of calcium carbonate, dolomite, or lime so as to cause chemical reaction with the sulfur dioxide and removal from the gases.
When calcium compounds such as lime or limestone are added as a slurry to the scrubber liquid, solid calcium salts may be formed. These salts have a limited solubility to form ions which react with the sulfur dioxide in the scrubber. Such wet calcium-containing scrubbing systems are therefore limited in efficiency by the solubility of calcium salts in the slurry. In addition, the formation of calcium sulfate in a supersaturated state in such systems creates a problem of scaling or deposition of inorganic solids in the scrubber and connected conduits which interferes with the operation of the sulfur dioxide removal system.
Many methods and additives have been used to reduce the oxidation of calcium sulfite to the scale-forming calcium sulfate in lime scrubbers. Such additives include magnesium oxide (U.S. Pat. No. 3,919,393), thiosulfate (U.S. Pat. No. 3,937,788) and hydroquinone (U.S. Pat. No. 3,485,581).
Further, U.S. Pat. No. 3,920,421 teaches the co-removal of SO.sub.2 and NO.sub.x (although not in a lime scrubber) by a catalytic process wherein the sulfite ions are oxidized to sulfate and nitrogen oxides are reduced to elemental nitrogen.
A report by Graefe, et. al. (PB196781, NTIS) entitled "The Development of New and/or Improved Aqueous Processes for Removing SO.sub.2 from Flue Gas", Volume II, discusses the effects of NO.sub.x on sulfite oxidation in, principally, sulfite-bisulfite scrubbers. However, the report teaches away from the present invention in that it categorizes both NO and NO.sub.2 as oxidizers, although it does disclose that NO.sub.2 is the more active. FIG. 6 therein shows that generally higher oxidation can be expected in a sulfite-bisulfite scrubber when NO is present (with or without NO.sub.2) than if NO is not present. Graefe, et. al. also show a process for eliminating sulfate in a dry fluidized bed absorber and reducing sulfate formation to a very low value in an aqueous absorber through use of a pre-scrubber to reduce NO.sub.2 by converting it to NO. The overall teaching, however, is distinguishable from the present invention which teaches that not only is NO not an oxidant but that by itself or in high enough proportion with NO.sub.2, it will act to inhibit oxidation.